Transmission line transition



1 Dec. 30, 1952 R. w. coRNx-:s 2,623,946

TRANSMISSION LINE TRANSITION Filed March 29, 194'? Fig. 4.

Y: Vf l///\\\\\\\\\\\\\\\\\\\\v INVENTOR Y am .COR/VES 26 Pfr/vp HW Patented Dec. 30, 1952 TRANSMISSION LINE TRANSITION Randolph W. Cornes, Garden City, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application March 29, 1947, Serial No. 738,193

9 Claims. 1

The present invention relates to the art including transitions or couplings between two transmission lines of diliering sizes. It is particularly applicable as a transition between two coaxial lines of differing sizes.

It is an object oi the present invention to provide an improved transition between two transmission lines of diiiering sizes.

it is another object of the present invention to provide between two transmission lines of diifering sizes, a substantially-reflectionless transition of comparatively small longitudinal dimension.

It is yet another object or" the present invention to provide a broad-band transition between two coaxial lines of differing sizes.

in the preferred embodiment of the invention described hereinafter, the transition couples two dilierent-sized transmission lines of the coaxial type, and comprises a pair of coaxial line transition sections in tandem between the differentsized 1lines to be connected. A thin flange serving effectively as a capacitance is located at the junction oi the inner conductors of these two transition sections, and its eiiective capacitance is correlated to the characteristic impedances of the lines to be connected, to the characteristic impedances of the transition sections, and to the lengths ci the transition sections, in the manner described hereinafter, to provide the advantages and to attain the objects of the present invention.

The invention aiso relates to the novel features or principles of the instrumentalities described herein, whether or not such are used for the stated objects or in the stated elds or combinations.

i is a schematic line drawing of a generalized transition oi the type used in the present invention and usieul in explaining the operation ci the present invention.

Fig. 2 is the equivalent circuit diagram ci the transition of Fig. l.

Wig.

3 is a graph useful in explaining the 0p- .ticn of the apparatus of Fig. 1`

Eig. 1; is a longitudinal-cross-sectional view oi one physical embodiment of the apparatus of Fig. l.

Referring to Fig. 1, a transition is shown schematically as connected between va first coaxial line end section 3 having an inner conductor 4 and an outer conductor 5, and a second coaxial Y line end section having an inner conductor 5 and an outer conductor 1 which are larger in diameter than the respective conductors of the lirst coaxial line end section 2. Joined in tandem to the first coaxial line end section 2 is a rst coaxial line transition section 8 having an inner conductor 9 and an outer conductor IEJ whose diameters are larger respectively than those of the inner conductor 4 and the outer conductor 5 of the first end section 2. Connected between the iirst transition line section 3 and the second end section 3, is a second coaxial line transition section Il, having an inner conductor l2 of diameter intermediate the diameter of the inner conductor 9 of the rst transition section 8 and the diameter of the inner conductor 6 of the second end section 3. Transition section Il also has an outer conductor I3 of the same diameter as the outer conductor 'l of the second end section 3 but of larger diameter than the outer conductor l0 of the iirst transition section 8. At the junction oi the inner conductor 9 and the inner conductor l2 of the two transition sections 8 and Il, there is positioned a thm disc-shaped conductive flange i4 which, as will be seen, determines the effective capacitive admittance at that point.

Dielectric beads I5 and I6 fill the inter-conductor spaces in the first and second transition sections 8 and ll, respectively. The dielectric beads l5 and I6, in addition to their effects on the characteristic impedances of the two transition sections 8 and Il, act as supports for the inner conductors 9 and l2 of the two transition sections.

Having thus described the general arrangement of the present invention, the manner in which it provides its advantages will now be discussed. As is well known, ,field discontinuities in a coaxial line are caused by changesin outerconductor diameter, inner conductor diameter, or diameters of both conductors. Such discontinuities are troublesome since they cause undesirable standing waves and energy reflection, leading to intolerable frequency sensitivity and losses. rIhe effects of such discontinuities may be calculated by considering each discontinuity as a lumped admittanceV shunted between the line conductors at the location of the discontinuity. For frequencies sufficiently low so that the transverse dimensions of the line are less than a half wavelength, which is the case for the recommended operating frequencies of the present novel transition, the discontinuity admittance is capacitive. The values of discontinuity capacity have been calculated for various changes in coaxial line dimensions, as set out in the article by Vlhinnery, Jameson and Robbins entitied, Coaxial-line discontinuities, in the Proceedings of the I. R. E. of November 1944.

As indicated above, the `specic embodiment of the recent invention described hereinis adapted for use between coaxial lines 2 and 3 of different sizes but equal characteristic impedances. However, the present invention is not necessarily limited to equal impedances. The following analysis applies to the use of equal impedances as well 'as to cases where the characteristic impedances' 'differ but slightly, in which event, for the design problem the characteristic impedance of each coaxial line is assumed to be equal' to the average of their true characteristic i-mpedances, without aifecting substantially theloperation of the transition.

Under these conditions, according to the present invention the change yof. conductor' diameters at the junction of the first endsection 2V` with the iirst transition section 3 is selected toY provide a discontinuity capacitance equal to that at the junction of the second transition section H with second end section S. Also, the radial dimension (or outer diameter) of thefiange Ill at the junction of the first transition section 8' with the second transition section H is chosen. to provide a.

discontinuity capacity at that junction 'equal to twice that at the other junctions.

For simplicity of designV` calculation and of construction, the same dielectric material isy used at iii and I5, so that the same dielectric constant occurs in both transition sections 8 and I l. Also, the lengths and characteristic impedan-ces ofthe two transition sections are made equal, by selecting their inner and outer conductors of the same diameter ratio.

Fig. 2 shows a schematic circuit diagram equivalent to the structure shown in Fig.. l. End. section 2 can be representedr by a lter I constituted by a transmission line of characteristic impedance Zo, and end section 3 is represented by a similar filter IV.

The first transition section S rand the second transition section I l may then be represented by nlters II and III in cascade, each filter comprising a transmission' line section of length 2l and of characteristic impedance Zoi the transmission line section being shunted. by the same capaci-ty C at each end thereof. The capacity C has a value equal to the discontinuity capacity at the junction between each end section with its adjoining transition section. The two capacities C Iat the junction between the two lters 1I and III provide a resultant capacity of double value, thereby being equal to the discontinuity capacity ofered by flange I4 of Fig. 1 under the con-ditions imposed. Thus, two similar symmetrical lters II and III in cascade represent the transition. Each symmetrical filter is somewhat similar to a 1r section at lower frequencies, but has a line section of length 2l therein. The entire apparatus of Fig. 1 is thus represented by lters I, II, III and IV in cascade.

Each of the lters II and III may be considered as formed of two half-filters, generally referred to as half-sections, each of which -consists of a length Z of transmission line L shunted at one end by a capacity C, as shown in Fig. 2A. The image impedance Z1 of each half section looking into the half section from the capacity end (as deiined, for example, at page 81 of Transmission Networks and Wave Filters by T. E. Shea, Van Nostrand Company, 1929) may for practical purposes be considered to be:

where A=wavelength, in the line iu=2rf where.y ,fr-frequency Zo1=characteristic impedance of the line L.

A graph` of Zr, versusv bl is shown in Fig. 3, in which UZ is plottedv along the X-axis and Zr is plotted alongthe Y-axis. In-asmuch as bl is proportional to frequency f (when l is constant) the graph. canalso be considered a plot of Z1 versus for fixed Z.

The graph shows that Z1 changes very little with variation in frequencyl from f=0 to f=A. Thus, if' the imageimpedance Z1 of the half section of Fig. 2A bechosen as equal to Zu at some point in that frequency band, the impedance mismatch over the band of frequencies from f= to f=A is comparatively small. This may be referred to as the low pass region of the image impedance.

The choice ofthe parameters of the half section nlter for matching purposes is facilitated by considering the image impedance Z1 of the half section as ,f approacheszero. Considering f approaching 0 as a limit, the image impedance Z1 'at f=0 may, for practical purposes, -be considered to' be:

ZI l..-

the dielectric. The parameters of the half sectionv are then. chosen so that where n is a mismatch factor depending upon the maximum permissible VSWR over the desired Iband of frequencies andhaving a maximum value `of 1. It will generally have. the value of .99 or slightly lower.

For greater bandwidth,` the, characteristic im- Dedance Zoi of the transition sections is made greater than the characteristic impedance Z0 of the end sections..

Inasmuch as each. transition section is a symmetrical filter consisting of two such. half-section filters and the entire transition consists of two such synimetrical filters, it can be seen that the overall design of the transition is obtainable from the parameters of the single half-section.

From Equation l it can be seen that the rate of change of Z1 with frequency is a function or" the rate of change of cot bl with frequency. If is large, the rate of change of cot ci with frequency is more rapid than with Z small. Therefore, the frequency band over which the variation of Z1 with frequency is comparatively small decreases with increasing axial length of the transition section. However, the axial length of the dielectric btatl in each transition section must be suincient to give proper mechanical support for the inner' conductor. The axial length of the transition section should therefore be a compromise between small length for broader' frequency band and large length for increased mechanical strength.

Fig. 4 shows the actual construction of one form of the invention embodying the principles discussed hereinabove. The transition unit is connected between a second coaxial line end section 23 and a smaller first coaxial line end section 22 which is terminated in a standard female fitting Sil. The transition unit comprises a nrst coaxial line transition section 28 having an inner conductor 29 and an outer conductor Sil whose diameters are larger respectively than those cf the inner conductor 24 and the outer conductor 25 of the smaller coaxial line end section 2E. Connected between the first coaxial line transition section 28 and the second coaxial line end section 23 is a second coaxial line transition section 3i having an inner conductor 32 of diameter intermediate the diameter of the inner conductor 29 of the nrst transition section 23 and the diameter of the inner conductor E@ of the second coaxial line end section E3. The second transition section 3i also has an outer conductor cl3 of the same diameter as the outer conductor 2l' of the second end section 23. At the junction of the inner conductor 29 and the inner conductor 32 of the two transition sections 23 and di, there is positioned a thin disc-shaped conductive flange Sill. Filling the interconductor spaces in the first and second transition sections 28 and 3! are respective dielectric beads 35 and 3d. Surrounding and joined to outer conductor 33 of the second transition section 3| is flange 5l. Equally disposed around fiange 3l are three dielectric pins S8, each of which is threadedly engaged with the flange 31 and extends into a cylindrical hole in the dielectric bead 3S. The dielectric pins 38 are composed of the saine material as the dielectric bead 36 and keep the dielectric bead 35 fixed with respect to the outer conductor S3 of the second transition section 3l. Inasznuch as dielectric bead 36 is fixed with respect to the outer conductor 33, the disc-shaped conductive flange 34, in addition to its electrical function of providing capacity, aids in holding in place the inner conductor 29 of the first transition section 2S and the inner conductor 32 oi the second transition section 3 I,

In the construction shown in Fig. the outer conductors of the smaller second en section 252, the rst transition section 2t, the second transition section 3i and the second end section are one integral subassembly. Two subassemblies 42 and lil make up the inner conductors of the four sections. The rst inner conductor subas sembly 62 is an integral unit formed of the inner conductor 2d and the inner conductor 32 of the second end section 23 and the second transition section 3l respectively, both inner conductors 2 and 32 being of hollow construction. The second subassembly iii is an integral unit formed of the inner conductor 29 and the inner conductor 2li of the first transition section 2E and the smaller end section 22. The second subassembly also includes the conductive flange 3d and a tubular internally-threaded portion d3 which is adapted to be inserted into the hollow inner conductor 32 of the second transition section ei.

In the assembling of the transition tre outer conductor subassembly. The second inner conductor subassembly 40 is then inserted through the hole in the dielectric bead 35, the

conductive flange 34 resting against the dielectric bead 35. The dielectric bead 36 is next inserted into position in the outer conductor subassembly. The first inner conductor subassembly is then inserted into position, the tubular projection 43 of the second inner conductor subassembly 4e projecting into the hollow inner conductor 32 of the rst inner conductor subassembly 42, and the inner conductor 32 resting against the conductive flange Sli. The two inner conductor subassemblies are held together by a screw 44 engaging tubular projection 43. The dielectric pins 38 are then inserted into the dielectric bead 36. To prevent the inner conductor subassemblies lill and 42 from rotating within the outer conductor subassemblies, the outer surfaces of the inner conductors 29 and 32 are knurled.

In one form actually constructed to yield the above mentioned advantages, the transition was designed to connect a 7/8" coaxial line end section to a coaxial line end section having a type -N- fitting termination. In the V8" coaxial line section, the outer diameter of the inner conductor Was .375 inch and the inner diameter of the outer conductor was .812 inch. In the coaxial line end section terminated in a type -N- tting, the outer diameter of its inner conductor was .120 inch and the inner diameter of its outer conductor was .271 inch. The various elements of the transition between the two end sections had the following dimensions:

A. 'First coaxial line transition section:

l. lOuter diameter of inner conductor- .131

inch 2. Inner diameter of outer conductor-.586

inch 3. Axial length-.405 inch B. Second coaxial line transition section:

1. Outer diameter of inner conductor- .181

inch 2. Inner diameter of outer conductor- .812

.inch 3. Axial length- .405 inch C. Conductive flange:

1. Outer diameter-.265 inch 2. Axial length-.015 inch The material forming both dielectric beads was a cross-linked condensation polymer of aniline and formaldehyde, sold under the trade name of Dielectene No. by the Continental-Diamond Fibre Company, Newark, Delaware. Its dielectric constant is 3.4.

The impedance mismatch caused by the transition having the 4above-mentioned dimensions was well within satisfactory limits over a frequency range of 0-4000 mc. The VSWR of the standing waves caused by the transition was below 1.1 over that frequency range. The transition as so constructed is particularly recommended for operation in the frequency range of GOO-3500 mc. It can be seen, therefore, that each transition section is much shorter than 1A; A at the center frequency of the band and the entire transition is Iappreciably shorter than a reasonably eicient tapered transition for that band of frequencies.

Inasmuch as a desired discontinuity capacity may be obtained by changes in inner conductor diameter alone, in outer conductor diameter alone, or in both inner and outer conductor diyarneters, the scope of the invention is not restricted to the particular changes shown but encompasses all types, of constructions that may be vusedto effect a desired discontinuity capacity. Also, inasmuch as the image impedance of a half section is a function of many parameters of the half section, the scope ofthe present invention encompasses all combinations of values of the different parameters of the half section that give a desired image impedance.

Although each transition section is described above as a symmetrical filter, it can be seen that impedance matching can also be eiiected in accordance with the teachings herein by transition Sections that are asymmetrical iter sections. Also, although the transition sections are described above as being physically similar sections, it can be seen that proper matching can also be effected by dissimilar sections.

In addition, although described above a transition between coaxial lines of dierent sizes but of substantially the same characteristic iinpedance, the invention is not limited to such transitions but, by proper design, is applicable as a transition between coaxial lines of diiering characteristic impedances.

Also, the scope of the invention is not limited to transitions between coaxial lines, but, by direct analogy includes transitions between other transmission lines of the hollow-internally-conducting type, such as waveguides, wherein -a desired discontinuity admittance may be effected by a proper change in line size or by elements positioned within the line.

rlhus, it can be seen that I have provided a novel transition between transmission lines that has wide application; that is compact; and that produces only a very low VSWR over an extremely broad frequency band.

Since many changes could be made in the above construction and many apparently v'i ely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A transition between two coaxial line-s of differing sizes comprising aV iirst transition section of coaxial line of size diiiering from one oi said coaxial lines and joined to said one of said coaxial lines whereby :a first discontinuity caa pacity exists at the juncture of said first transition section and the said one of the coaxial lines; a second transition section of coaxial line joined to said first transition section and of size diiiering from the size of the second of said coaxial lines, said second transition section of coaxial line being joined to said second of said coaxial lines whereby a second discontinuity capacity exists at the juncture of said second section and the second of said coaxial lines, the juncture of said iirst and second sections providing a dis continuity whose discontinuity capacity is substantially equal to the sum or" said first and second discontinuity capacities.

2. Apparatus as in claim 1 in which said second discontinuity capacity is equal to said first discontinuity capacity.

3. Apparatus as in claim l in which the axial lengths of said lirst section and said second section are equal.

4. Ultra high frequency apparatus comprising a first coaxial line section including an inner conductor and an outer conductor spaced therefrom; a second coaxial line section having an inner conductor joined to the said inner conductor and an outer conductor joined to said outer conductor, at least one of said conductors of said second section of coaxial line differing in size from its corresponding conductor of said iirst coaxial line section; a third coaxial line section including an inner conductor and an outer conductor spaced therefrom, said latter inner conductor being joined to said second section inner conductor and said latter outer conductor being joined to said second section outer co-nductor, at least one of said conductors of said third coaxial line section diifering in size from its respective conductor of said second coaxial line section; a capacity element at the juncture of the inner conductor of said second coaxial line section and the inner conductor of said third coaxiavl line section; and a fourth coaxial line section having an inner conductor and an outer conductor, said latter inner conductor and said latter outer conductor being joined respectively to the inner conductor and outer conductor of said third section of coaxial line, one of said conductors of said fourth section differing in diametral dimension from its corresponding conductor of said third coaxial line section.

5. Apparatus as in claim 4 further including solid dielectric material iilling the space between the inner conductor and the outer conductor of said second coaxial line section, and also including solid dielectric material filling the space between the inner conductor and the outer conductor of said third coaxial'line section, whereby said solid dielectric materials aid in supporting said inner conductors.

6. Ultra high frequency apparatus comprising a iirst coaxial line section including a first innre conductor and a iirst outer conductor, and having a predetermined characteristic impedance; a second coaxial line section Iincluding a second inner conductor whose diameter is larger than the diameter of said rst inner conductor joined to said first inner conductor, said second coaxial line section also including a second outer conductor whose diameter is larger than the diameter of said first outer conductor and joined to said iirst outer conductor, said second coaxial line section further including solid dielectric material filling the space between said second inner conductor and said second outer conductor, whereby a rst discontinuity capacity exists at the juncture of said rst coaxial line section and said second coaxial line section; a third coaxial. line section having a third inner conductor and a third outer conductor joined respectively to said second inner conductor and said second outer conductor, said third inner conductor and said third outer conductor having diameters larger than the respective diameters of said second inner conductor and said second outer conductor, said third coaxial line section also in- Cluding solid dielectric material filling the space between the said third inner conductor and said third outer conductor; a flange at the junction. of said second second and third inner conductors and providing a second discontinuity capacity whose value is substantially twice that oi said first discontinuity capacity; and a fourth coaxial line section whose characteristic impedance is substantially equal to the characteristic impedance of said nst coaxial line section and includ ing fa fourth outer conductor joined to said third outer conductor section, the diameter of said fourth outer conductor being equal to the diameter of the said third outer conductor, said fourth coaxial line section also including a fourth I 9 inner conductor joined to said third inner conductor, the diameter o-f said fourth inner conductor being larger than the diameter of said third inner conductor and having at their junction a third discontinuity capacity substantially equal to said iirst discontinuity capacity.

7. Coaxial line apparatus comprising an inner conductor having a predetermined diameter for a first axial extent of said coaxial line, an enlarged diameter over a second axial extent of said coaxial line contiguous to said first axial extent, a still greater enlarged diameter over a third axial extent and contiguous to said second axial extent and equal in length to it, and greatest diameter for a fourth axial extent contiguous to said third axial extent; an outer conductor having a predetermined diameter for said first extent, an enlarged diameter for said second axial extent, and a still further enlarged diameter for said third and said fourth axial extents; solid dielectric material lling the space between said inner conductor and said outer conductor in said second and said third axial extents; and a ange element on said inner conductor at the junction of said second and third axial extents; the ratio of the diameters of said inner conductor and said outer conductor over the said fourth axial extent being substantially equal to Ithe ratio of the diameters of said inner conductor and said outer conductor over said first axial extent, whereby the characteristic impedance of said fourth axial extent of said coaxial line is substantially equal to the characteristic impedance of said rst axial extent of line, the diameter oi the inner conductor in said third axial extent of said coaxial being so related to the diameter of the inner conductor in said fourth axial extent line as to produce a rst discontinuity capacity substantially equal to the second discontinuity capacity due to the discontinuity existing between said second and said rst axial extents of said coaxial line, the radial dimension oi said flange being such as to produce a third discontinuity capacity substantially equal to twice said first or second discontinuity capacity.

8. Apparatus as in claim 7 in which the image impedance Z1, of said second line extent looking into said second axial extent of said coaxial line from said first axial extent, is substantially equal to the average of the characteristic impedances of said first and fourth axial extents of said coaxial line at a point in the low pass frequency region of said image impedance, where b=21r divided by wavelength i at said point W=21r times frequency f at said point Zni=the characteristic impedance of said second axial extent of said coaxial line Czdiscontinuity capacity due to the discontinuities between said rst and second axial extents of said coaxial line lzone-half the axial length of said second axial extent of coaxial line.

9. Ultra high frequency apparatus comprising first, second, ythird and fourth transmission line sections in tandem connection for energy transfer between said rst and fourth sections through said second and third sections, each of said iirst, second, third and fourth sections having an axial inner conductor and an outer conductor, the inner and outer conductors of said first transmission line section being of appreciably smaller transverse dimensions than the inner and outer conductors of said fourth transmission line section, each of said second and third transmission line sections being differently dimensioned transversely in at least one conductor than the adjacent transmission line sections whereby a transverse dimensional step exists at each junction between successive transmission line sections, resulting in a shunt capacitance concentration thereat, said second and third transmission line sections each comprising a coaxial transmission line section wherein the inter-conductor space is nlled with solid dielectric material, radially extending conductive means at the junction between said second and third transmission line sections providing a shunt capacitance concentration thereat appreciably exceeding the capaciytance concentrations at each of the other two junctions in said apparatus.

RANDOLPH W. CORNES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,841,473 Green Jan. 19, 1932 1,921,117 Darbord Aug. 3, 1933 2,183,123 Mason Dec. 12, 1939 2,267,371 Buschbeck Dec. 23, 1941 2,403,252 Wheeler July 2, 1946 2,405,437 Leeds Aug. 6, 1946 2,406,372 Hansen Aug. 27, 1946 2,432,093 Fox Dec. 9, 1947 2,449,570 Violette Sept. 21, 1943 FOREIGN PATENTS Number Country Date 895,883 France Feb. 6, 1945 OTHER REFERENCES Proceedings of the I. R. E., volume 32, No. 11, November 1944, pages 695-709. 

